1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus which allows imaging of a large region of a subject by acquiring magnetic resonance (MR) signals while moving the subject in the direction of its body axis.
2. Description of the Related Art
The MRI method is an imaging method which involves exciting nuclear spins in tissues of a subject placed in a static magnetic field with a radio-frequency signal (RF pulses) having their Larmor frequency and reconstructing image data from magnetic resonance signals emitted as a result of the nuclear spins having been excited.
The MRI apparatus is an imaging diagnostic apparatus which allows imaging of a subject through the use of the MRI method. The MRI apparatus can obtain not only anatomical diagnostic information but also biochemical information, diagnostic function information, etc. For this reason, the MRI apparatus plays an increasingly important role in the field of today's imaging diagnosis.
FIG. 4 is a schematic illustration of the gantry of a conventional MRI apparatus. Around an imaging field 130a set in the center of the gantry 120a are arranged a main magnet 11a, a gradient field coil 21a, a transmit coil 31a and a receive coil 33a. The main magnet 11a and the gradient field coil 21a form a static magnetic field and gradient magnetic fields, respectively, to which a portion of a subject 150 placed in the imaging field is subjected. The transmit coil 31a applies the subject 150 with RF pulses. The receive coil 33a detects MR signals emitted from the subject as a result of application with the RF pulses.
A top board (i.e., a support) 4a on which the subject 150 is placed is mounted to the top of an examination couch (not shown) so as to be slidable in the direction of its length (the Z-axis direction). When the subject 150 is moved into the imaging field together with the top board 4a, a portion of the subject (a to-be-imaged region) is placed in the proximity of the receive coil 33a. In this case, the maximum imaging region is determined by the length D in the direction of Z axis of the receive coil 33a. 
In recent years, to produce serial image data for a region which is even larger than the maximum imaging region, a method has been proposed which produces image data by reconstructing MR signals acquired while continuously moving the top board 4a in the direction of its length (see, for example, JP-A 8-71056 (KOKAI)).
According to the method proposed in this publication, by controlling the carrier frequency of selective excitation RF pulse to a given slice plane of the subject 150 in acquiring MR signals, MR signals from given to-be-imaged region of the subject in movement can be captured at all times by the same frequency, allowing image data for a large region of the subject to be produced.
In addition, an imaging method has also been proposed which allows multiple-slice imaging or oblique imaging as a result of further development of the above method (see, for example, JP-A 2002-95646 (KOKAI)).
As described already, the MRI apparatus applies to the subject 150 RF pulses having frequency components in a given band (for example, ±250 KHz) centered at a Larmor frequency of, say, 63.9 MHz and then receives from the subject MR signals having the same frequency components as the RF pulses to produce image data. The MR signals are signals of very low amplitude; therefore, it is required to remove noise having the same frequency components as much as possible. For this reason, the receive coil 33a to detect MR signals and the gantry 120 equipped with amplifiers to amplify the detected MR signals are installed in a shield room together with the examination couch.
In the production of image data for a large region of a subject which is continuously moved (large-volume imaging), there is a fear that electromagnetic noise caused by a drive signal to move the top board 4a gets mixed in MR signals detected by the receive coil 33a and consequently unacceptable artifacts are produced in image data obtained by reconstructing the MR signals.
In particular, with MRI apparatus structured such that the length D of the imaging field 130a of the gantry 120a is reduced and its aperture is made large in order to remove an uneasy feeling of the patient 150 moved into the imaging field, the receive coil 33a and the amplifiers are more susceptible to electromagnetic noise.
Besides the drive signal to move the top board 4a, there are other causes which allow electromagnetic noise to get mixed in MR signals.